Yellow submarine goes missing off Qld

But unlike the one immortalised in the 1960's Beatles hit, nobody lives in this one.

And nobody ever will - it's far too small.

So small, in fact, that the scientists who have lost it hope it may get washed ashore or perhaps turn up in a fisherman's net.

The $500,000 unmanned research vessel, technically a remote-controlled Integrated Marine Observing System named AUV Sirius, is operated by the University of Sydney's Australian Centre for Field Robotics (ACFR).

ACFR Associate Professor Stefan Williams said it was being used last weekend by scientists from the University and the CSIRO to map kelp and coral on the sea-floor east of Moreton Island as part of a larger national mapping program.

Prof Williams said AUV Sirius had completed its mission on Saturday, surfaced and shut down. But before it could be hooked up to the recovery vessel, something went wrong and it suddenly dived out of sight.

"We think either a pressure housing flooded or a change in water density caused it to sink," he said in a statement.

Professor Williams said the bright yellow vessel, 1.5 metres wide and two metres long, is fitted with a radio beacon and an emergency strobe.

"These have enough battery power to last one week. After that, our only hope is Sirius comes to the surface again, or is washed ashore or pulled up in a net," he said.

Coast Guard Brisbane conducted an extensive sea and air search last weekend and several days this week, covering hundreds of square nautical miles of ocean, without success.

The Coast Guard believes AUV Sirius is drifting south and could now be off the Gold Coast or upper Tweed.